Processing Method And Propagation Characteristics Of Airgun Source Signals

The research on regional-scale continental subsurface structure,the state,and change of medium is a common focus of earthquake seismology and exploration seismology.Developing a new artificial seismic source best for the detection of regional-scale continental shallow subsurface structure and the monitoring of medium temporary change is a foundational tool to build seismic radar platform and actively transmit seismic wave.It will provide a new method for deep hydrocarbon exploration,shallow geodynamics,urban geophysics,earthquake prevention and disaster mitigation.Presently,the main sources used for the dynamic monitoring of medium change are similar earthquake,background noise,artificial source and some unconventional sources.Among the natural sources: spatial and temporal distribution of natural earthquakes is uneven,similar earthquake has a limited occurring frequency,seismic background noise varies seasonally and needs a long time stacking to obtain the stable measurements.Due to the above factors,the spatial and temporal resolution and precision of medium change monitoring with these sources are limited.Among the artificial seismic sources: the traditional explosive source is restricted in application due to safety reasons,while the vibroseis is not suitable for regional-scale continental monitoring due to weak energy release.In order to select an ideal source,we carry on a series of field experiments to test a variety of seismic sources and ultimately choose the un-tuned large volume airgun array operated in on-land reserviors as the preferred source.Airgun has been proved to be an environmental-friendly,efficient,and highly repeatable source.In this dissertation,combined with the core purpose of regional-scale continent crust structure detection and dynamic monitoring,we study problems related to the airgun source signal processing technologies and propagation characteristics on top of the previously conducted un-tuned large volume airgun array excitation experiments.The main work of this thesis includes the following aspects:Firstly,we introduce a comprehensive review and comparison of large volume on-land airgun array excitation experiments.Based on the review and summary of the airgun history,technical principles and development of the airgun excitation theory,we discuss the differences between marine airgun source and on-land airgun source.The on-land airgun source in limited water is the coupling result of airgun array and the water environment.It is a physical gas-liquid-solid three-phase coupling system involving bubble oscillation theory,the coupling of gas-liquid and liquid-solid interface,water boundary conditions,and complex mechanism of seismic wave propagation.The solid-liquid interface is the key characteristics of the on-land airgun.Then we introduce the large volume airgun source experiments in last decade in details,summarize the unique advantages of airgun source in regional-scale medium change monitoring and compare the signal character and factors that influence experiments results.The differences between airgun signal simulation and actual signal observed show that the main factors consist of airgun body?type,capacity,and synchronization?,working conditions?excitation pressure and depth?and water conditions?water level,shape and size of reserviors?.A summary of the influence of different factors is helpful to deepen the understanding of the on-land airgun source focal characteristics,improve the excitation environment design and evaluation and obtain a better effect.Secondly,we propose an automatic airgun data filter and processing method.Noise level analysis shows that the noise level of stations around Binchuan in the 2-10 Hz brand is generally around-130 d B,only a few stations higher than this level.The noise level of stations affects the processing of airgun signal.In order to eliminate the influence of small seismic events and strong noise to the stacking effect of airgun signal,we propose a new method based on noise characteristics,Root Mean Square?RMS?filter method.When processing the airgun signal,the method of stacking multiple low excitation energy to mimic a large energy excitation and utilizing the interferometric to monitor medium changes requires stacking techniques;massive dataset from airgun signal transmitting stations requires automatic processing method.Practical application results show that the RMS filtered linear and time-frequency domain phase weighted stacking are superior to the conventional methods.It can suppress the strong noise,effectively improve the signal-to-noise ratio?SNR?and increase detection distance.The RMS method increases the Binchuan station signal's propagation distance from 250 km to 350 km,Hutubi station from 500 km to 1000 km and Zhangye station from 300 km to nearly 500 km.Lastly,we research on wavefield propagation characteristics and differences of airgun sources.A series of problems related to wave propagation are studied with datasets from three airgun signal transmitting stations.On the basis of analyzing the signal energy space distribution and spreading direction differences,we study the airgun Pn phase attenuation beneath middle Tianshan region and obtain Q₀= 388,which is consistent with the previous results from earthquakes.This shows using airgun source to study regional phase attenuation is feasible.The velocity strength of each airgun source is around 10⁶nm/s order of magnitude at its corresponding reference station.The velocity strength from Hutubi is the strongest,while the signal attenuation from Binchuan station is the strongest due to the complicated geological structure in Yunnan area.On the other hand,the study of S wave amplitude distribution shows S wave of airgun source might be generated from the impact and coupling of airgun on the water bottom and shore.Research on the affect of airgun source signal propagation helps to understand the on-land airgun source excitation mechanism and explore the potential applications in different areas.